1. Field of the Invention
This invention relates to an .alpha.-cyanoacrylate-type adhesive composition, and more particularly to an adhesive composition having a fast setting time.
2. Description of the Prior Art
.alpha.-Cyanoacrylate-type adhesives, because of the inherent anion-polymerizability of the .alpha.-cyanoacrylate monomer as a main component, begin to polymerize in the presence of a weak anion such as in the presence of slight moisture adhering to the surface of the adherend, and can firmly bond almost all materials except certain inert materials such as polyethylene and tetrafluoroethylene resins (e.g., Teflon) within several seconds to several minutes. Hence, they have found extensive use as instantaneous adhesives in industry, in medical uses, in hobby applications, and in the home.
The curing of .alpha.-cyanoacrylate-type adhesives is due to the anionic polymerization of the .alpha.-cyanoacrylate monomer. When such an adhesive is used to bond materials whose bonding surfaces are acidic, such as wood or surfaces which tend to permit the formation of an acidic oxide coating, the anionic polymerization of the .alpha.-cyanoacrylate is inhibited, and the setting time of the adhesive is slow. Thus, the adhesion strength is sometimes not entirely satisfactory. For example, wooden materials in general use have a moisture content of about 10% by weight under normal conditions (i.e., at 23.degree. C. and a relative humidity of 55%). In spite of this high water content in the woody tissue and on the surface of the wooden materials, a setting time of from several minutes to between ten and twenty minutes is required to bond these wooden materials with conventional .alpha.-cyanoacrylate-type adhesives. Since .alpha.-cyanoacrylate-type adhesives penetrate into the woody tissue during this period, it is generally considered difficult to bond wooden materials with these adhesives. Usually, wooden materials are acidic because of the presence of sap, etc. in the wood. If the surface of the material is acidic, naturally the anionic polymerization of the .alpha.-cyanoacrylate monomer would be hindered, and the speed of curing decreased. Consequently, the effect of the adhesives as instantaneous adhesives decreases drastically.
A chromate-treated surface of a metal is usually acidic, and a surface obtained by neutralizing the acidic surface using a Unichrome treatment has lower weatherability than the chromate-treated surface. Accordingly, where parts with weatherability are required, such preferably have an acidic chromate-treated surface. In bonding such a surface, .alpha.-cyanoacrylate-type adhesives have a slow setting time, and the adhesion strength of the bond is low.
.alpha.-Cyanoacrylate-type adhesives are chemically reactive adhesives which cure by anionic polymerization. These adhesives usually have a low viscosity, and therefore, when they are applied to the adherend, an initial tackiness, as in the case of rubber-type adhesives, is not obtained with .alpha.-cyanoacrylate-type adhesives. The adherend must be fixed for several minutes until anionic polymerization begins.
Furthermore, in a bonding operation on an assembly line, the adherend must be fixed for a certain period of time by a jig, etc., and therefore, the efficiency of operation decreases. Since .alpha.-cyanoacrylate adhesives are "instantaneous" adhesives, it is necessary for them to cure on many materials with as fast a setting time as possible. The rapidity of the setting time will broaden the range of application of .alpha.-cyanoacrylate-type adhesives.
Many methods for increasing the setting time of .alpha.-cyanoacrylate-type adhesives have been investigated heretofore. These methods include, for example, a method which involves increasing the purity of the .alpha.-cyanoacrylate monomer, and a method which involves decreasing the amount of anionic polymerization inhibitor to be added.
Generally, the activity of anionic polymerization tends to be markedly affected by very small amounts of impurities, and in anionic polymerization of an .alpha.-cyanoacrylate monomer, an attempt has been made to minimize the amounts of impurities such as the presence therein of starting materials, catalyst and polymers of low degrees of polymerization in order to increase the setting time of the resulting product. However, because an .alpha.-cyanoacrylate monomer has unique anionic polymerizability, difficulties, such as the provision of a feasible apparatus, are encountered in obtaining an .alpha.-cyanoacrylate monomer of a high purity. Accordingly, this method of increasing the purity of the monomer is difficult to perform on a commercial scale.
Usually, an anion polymerization inhibitor (stabilizer) such as sulfur dioxide, propane sultone or p-toluenesulfonic acid is incorporated in .alpha.-cyanoacrylate-type adhesives to inhibit the anionic polymerization of the monomer due to the presence of water, etc. in the container during storage. The addition of the inhibitor serves to inhibit the anionic polymerization of the monomer during storage, but retards the setting time of the adhesive. An attempt has therefore been made to decrease the amount of the anionic polymerization inhibitor and thereby to increase the setting time. This approach, however, is limited in application because a reduction in the storate stability of the adhesive occurs as well.